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HDMI 2.0 的 ESD 防护

08 October 2020 / by Anindita Bhattacharya

Anindita Bhattacharya

Semtech_TVS_ESDforHDMIDuring the ongoing Coronavirus pandemic when we are mostly confined to our homes, we are spending an increased amount of time in front of screens. This includes attending online meetings, taking video calls, playing video games, watching television, and also working longer hours on our computers. A high quality audiovisual (AV) experience is a must these days and thanks to the high definition digital displays and high definition multimedia interfaces (HDMI), we are able to experience high speed, high resolution AV with higher pixel depths and greater aspect ratios.

HDMI 是一种数字接口,用于将高清、高速数字多轨音频和未压缩视频信号从 HDMI 兼容源传输到其他视听显示器。自 2002 年 HDMI 发布以来,它在消费电子产品中得到了广泛的应用。今天,HDMI 连接计算机、游戏机、蓝光/DVD 播放器、电视、投影机、家庭娱乐系统、显示监视器和许多其他视听设备。

HDMI System Architecture & Pin Configurations

HDMI 系统架构由一根电缆以及源端和接收端的连接器组成,如图 1 所示。高分辨率未压缩视频、音频和控制信号通过一根 HDMI 电缆传输。我们来分析一下系统架构中数据如何从源传输到接收器。

Semtech_Diagram_HDMISystem

Figure 1. HDMI System Architecture

The transition minimized differential signaling (TMDS) algorithm is used to transmit the video and audio signals from the transmitter to the receiver. This algorithm uses four differential pairs. Three of them are TMDS data channels and one is the TMDS clock channel. Each TMDS data channel is composed of data+, data- and data shield. The TMDS clock channel is composed of clock+, clock- and clock shield. TMDS channels are synchronized with reference to the clock channel.

显示数据通道 (DDC) 是 HDMI 兼容设备和源之间的双向通信协议,允许源读取外围设备的标识数据。它提供型号、序列号、长宽比等信息。DDC 总线基于内部集成电路 (I2C) 总线。它由串行时钟或 SCL、串行数据或 SDA 线以及接地连接组成。

The Consumer Electronics Control (CEC) is a bidirectional bus to control or connect other CEC-capable devices from one source. CEC contains a single wire and a ground that it shares with DDC and HEC.

+5V 电源线用于在未开启电源时为接收器的 DDC 总线供电。例如笔记本电脑通过 HDMI 连接器连接到投影机。即使投影机没有打开,+5V 电源线也会为投影机的 DDC 线供电。这样就便于通过笔记本电脑读取投影机的信息。

热插拔检测 (HPD) 和 HDMI 以太网通道 (HEC) 数据+ 引脚共用同一条线路。HPD 允许信号源检测是否有任何支持 HDMI 的设备在运行时连接或移除。HEC 用于设备之间的以太网兼容数据网络。

图 2. HDMI Type A 连接器的引脚配置

图 2. HDMI Type A 连接器的引脚配置

图 2 显示了标准 A 型 HDMI 连接器的引脚配置。它包含 19 个引脚,包括四个 TMDS 对和其他六个通道(+5V 电源、HPD、CEC、SDA、SCL、HEC),以实现数字数据转换。自 2002 年推出以来,HDMI 已发展出许多规格,但 HDMI 2.0 仍然是最受欢迎和使用最广泛的接口。

HDMI 2.0 接口的 ESD 防护

HDMI 2.0 is designed to augment the AV experience for viewers by increasing the bandwidth significantly from previous versions. The maximum bandwidth of a TMDS channel is 6Gbps. The three TMDS channels are capable of total 18Gbps differential data transfer with resolutions up to 4K at 60 frames per second. They can deliver digital data up to two video streams and up to four audio streams. As consumers expect uninterrupted video and also plug and unplug HDMI connectors frequently, the EMC robustness of the HDMI interface is critical. These plug-unplug events can cause ESD events that can reach up to thousands of volts potentially damaging the electrical circuit. The TVS diode protects data ports from ESD threats by arresting the system-level ESD peak within a nanosecond, thereby diverting high currents away from the data port.

现在我们来探讨如何保护 HDMI 端口免受任何 ESD 事件的影响。图 2 所示为 HDMI A 型端口,其中 19 个引脚/通道可分为高速通道和非高速通道。高速通道包括 TMDS 通道(引脚 1-2-3;4-5-6;7-8-9)和 TMDS 时钟通道(引脚 10-11-12)。这些通道承载总计 18Gbps 的差分数据信号。

The maximum operating voltage on each differential line is 3.3V. Since these are extremely high-speed differential data lines, TVS diodes need to protect the circuit during transient events by maintaining an extremely low line-to-line capacitance while ensuring the signal integrity. As the TVS diode and the electrical circuit are connected in parallel, during an ESD event, the voltage drop across the circuit is same as the clamping voltage of the TVS diode. It is vital to keep the clamping voltage as small as possible. Therefore the protection device should include features like low ESD peak clamping voltage and low dynamic resistance. Additionally, the signal integrity of the high-speed interface can be optimized by choosing TVS devices that feature a flow-through routing package. This is extremely important to avoid signal distortion.

Semtech 的 RClamp®0574P 专为保护 HDMI 2.0 接口的高速差分线路而设计。它具有 0.18pF 的典型结电容,可保护四条高速数据线。它的工作电压为 5.5V,最小击穿电压为 6.5V。RClamp0574P 根据 IEC 61000-4-2 (ESD) 规范在 ±15kV(空气)和 ±10kV(接触)下提供瞬变保护。它的动态电阻为 0.26Ω(典型值)。RClamp0574P 采用 5 引脚 DFN 封装,标称尺寸为 2.0 x 1.0 x 0.5mm。由于一个 RClamp0574P 可以保护两个差分数据对,因此可以使用两个 RClamp0574P 来保护三个 TMDS 通道和一个 TMDS 时钟通道,如图 3 所示。该器件的流通式封装设计简化了 PCB 布局并有助于保持信号完整性。

Now let us see how to protect the rest of the six channels which are non-high-speed. These channels include CEC (Pin 13), HEC (Pin 14, 19), SCL (Pin 15), SDA (Pin 16), +5V power (Pin 18), and HPD (Pin 19). These channels can be protected by RClamp0574P as well, however, since these channels do not transmit high-speed data, low capacitance is not as critical for their protection during transient events. To protect these channels, a six channel TVS diode array is required with an operating voltage of 5V, fast response time, low clamping voltage, and no device degradation during the transient event.

Semtech’s RClamp7538P is perfectly suited to safeguard these six channels. It has a working voltage of 5V with peak pulse current of 5A. RClamp7538P provides transient protection as per the specification in IEC 61000-4-2 (ESD) at ±25kV (Air), ±20kV (Contact). It has a typical dynamic resistance of 0.42Ω. It comes in a nine-channel DFN package of size 3.8 x 1.0 x 0.5mm. The state-of-the-art flow-through scheme is also used for the package design which simplifies the PCB layout using this part. Refer to the connection diagram in Figure 3 below.

图 3. HDMI 2.0 连接的可能 ESD 保护选项

图 3. HDMI 2.0 连接的可能 ESD 保护选项

Summary and Conclusion

充分保护 HDMI 端口免受所有 ESD 威胁,这一点至关重要。谁都不希望计算机显示器或家庭娱乐系统因 ESD 事件而损坏。Semtech 值得信赖的高效 TVS 产品为世界上许多最受欢迎的显示设备提供保护。这些解决方案是保护 HDMI 2.0 接口的理想选择——让消费者连接高保真视听设备并享受流畅、不间断的体验。

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主题:电路保护

Anindita Bhattacharya

作者 Anindita Bhattacharya

Anindita Bhattacharya is a Solutions Architect of System Protection at Semtech’s Protection Products Group. She is also an adjunct faculty at San Jose State University.

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